| Yanqi Basin is located the southern piedmont of the middle Tianshan Mountains in Xinjiang province, situated in Bayingolin Mongolian Autonomous Prefecture. Yanqi Basin is a rhombus basin(high in northwest and low in southeast) surrounded by mountains. The surrounding mountains tilt from the basin margin to its center, the Bosten Lake(the largest inland freshwater lake in China). Groundwater is the dominant water supply source of domestic water, production water and ecological water in the Yanqi Basin. With the rapid development of industrial and agricultural production and urbanization, there is an intense contradiction between water supply and demand. Due to large-scale human activities in recent decades, hydrogeological problems, such as groundwater quality deterioration and decrease of groundwater table, tend to become increasingly serious. Our work is focused on how to solve these problems and use groundwater rationally. Further study of groundwater quality,groundwater evolution characteristics and hydrogeochemical processes is of great significance in reasonable exploitation of groundwater resources and protection of the groundwater environment.Groundwater in four counties(namely, Yanqi, Bohu, Hejing and Heshuo) located in plain area in the Yanqi Basin was taken as our research target. With a combination of comprehensive analysis of hydrochemical data and hydrogeological conditions, systematic research(including groundwater quality, hydrogeochemical characteristics, hydrogeological processes and assessment of phreatic vulnerability) was conducted using multi-methods(including inverse hydro-geochemical modeling, evaluation of groundwater quality and pollution and assessment of phreatic water vulnerability). Conclusions were reached as follows:(1)Chemical type of groundwater transformed from HCO3-Ca type(in 2011) to SO4 · Cl-Na type(in 2014) in the Yanqi Basin. In 2014, chemistry type of groundwater showed that the order of the change was HCO3, SO4 and then to Cl type from piedmont alluvial-pluvial fan to the Bosten Lake. The chemical types of groundwater evolved from HCO3-Ca type in the piedmont clinoplain to HCO3· SO4- Na·Ca type in the fine soil plain and then to Cl ·SO4-Na·Ca type in the lakeshore region in the Yanqi Basin. From 1999 to2014, plain areas with groundwater of HCO3-Ca type and HCO3·SO4- Na·Ca type extended133.5 km2 and 501.5 km2, respectively. While plain areas with groundwater of Cl ·SO4-Na·Ca type reduce 1537.6 km2.(2)There was a significant difference of groundwater TDS between 1999 and 2014(p<0.01). There was a significant positive correlation between TDS and Cl- and K++Na+. The correlation degree between TDS and major anion from high to low was Cl-, SO42-and HCO3-;the correlation degree between the TDS and major cation from high to low was K++Na+, Ca2+and Mg2+. Major ions of groundwater originated from weathering and dissolution of minerals in plain area in the Yanqi Basin. In time scale, the average TDS saw a general variation of an increase before a decrease followed by another increase before decrease during 1983 and2014. In spatial scale, the area of groundwater with TDS <1 g·L-1 increased from 2011.7 km2(in 1999) to 2229.3 km2(in 2014).(3)The results of quality evaluation of 42 groundwater samples in 2014 showed that the phreatic water quality of four monitoring wells were good(meet the groundwater quality standard of Grade III). There were two shallow confined groundwater samples of grade V(account for 18.1% of 11 shallow confined groundwater samples and 4.8% of the total 42 groundwater samples, respectively). There were six deep confined groundwater samples of grade V(account for 22.3% of 27 deep groundconfined water samples and 14.3% of the total42 groundwater samples, respectively). Quality of phreatic water was better than that of shallow confined groundwater which was better than that of deep confined groundwater in plain area in the Yanqi Basin in 2014.(4)Groundwater organic pollutants(including 39 kinds of pollutants) were detected for the first time in the study area. Results showed that there were three organic contaminants detected in groundwater in plain area in the Yanqi Basin with detection rate of 31.0%(trichloromethane), 2.4%(1, 2-dichloroethane) and 2.4%(1, 2-dichlorobenzene), respectively.Concentrations of the three compounds detected were under the drinking water standards.Results of groundwater organic pollution assessment showed that the organic pollutants did not have significant hazards to the environment in plain area in the Yanqi Basin.(5)The results of pollution evaluation of 42 groundwater samples showed that the pollution degree of phreatic water was classified as unpolluted and slight polluted. There were three unpolluted phreatic water samples and one slight polluted phreatic water sample(account for 75.0% and 25.0% of four phreatic water samples, respectively). The pollution degree of shallow confined groundwater was classified as unpolluted and slight polluted.There were nine unpolluted and two light polluted shallow confined groundwater samples(account for 81.8% and18.2% of 11 phreatic water samples, respectively). The pollution degree of the deep confined groundwater was classified as unpolluted, slight polluted and medium polluted. There were 20 unpolluted, five light polluted and two medium polluted deep confined groundwater samples(accounts for 74.1%, 18.5% and 7.4% of 27 deep confined groundwater samples, respectively). The degree of pollution of the deep confined groundwater exceed that of the phreatic water, and the degree of pollution of the shallow confined groundwater exceed that of the phreatic water in plain area in the Yanqi Basin in2014.(6)Inverse hydrogeochemical modeling of a typical profile was used to simulate the hydrogeochemical processes(including hydrolysis, evaporation, cation exchange and oxidation-reduction) which decided the chemical characteristics of the plain area in the Yanqi Basin. Cation exchanging gradually increased from the phreatic water to the deep confined water.(7)DRAV model was used to evaluate phreatic water vulnerability in plain area in the Yanqi Basin. Results showed that the area of extremely high vulnerability zone(with vulnerability index greater than 8.0), high vulnerability zone(with vulnerability index between 8.0 and 6.0), medium vulnerability zone(with vulnerability index between 6.0 and4.0) and low vulnerability zone(with vulnerability index between 4.0 and 2.0) accounted for2.5%, 21.5%, 49.3% and 26.7% of the total area, respectively. The vadose lithology was consisted of sand and gravel in high and extremely high vulnerability areas. |
PDF Full Text Request